There are many uses for sealed, pressure-balanced enclosures in the offshore industries, such as for housing sensors, batteries, and other sorts of apparatus that must be deployed at depth while not being directly exposed to the seawater itself. As one practical example, such enclosures are frequently used as subsea junction boxes. Components are typically joined within such junction-boxes while they are open. The enclosures are then closed and sealed prior to submersion in order to prohibit the intrusion of seawater. There are two broad existing categories of subsea junction boxes. One category comprises a gas-filled compartment in which various components are joined. The compartment pressure remains substantially at one atmosphere. This category of junction box must have very heavy walls and high-pressure seals to withstand the enormous pressure at great depth. In a second existing category of subsea junction boxes, the enclosure in which various components are joined, hereinafter called the termination chamber, is filled with oil that is pressure-balanced to the ambient sea pressure by a compensator. The walls and seals of these pressure-balanced junction boxes can be much less robust than those of the gas-filled junction boxes because there are no high-pressure differentials between the termination chamber and the exterior environment.
Subsea junction boxes are often used to join two or more subsea cables; or to join one or more cables to underwater connectors, or to other devices; or to join two or more devices without cables. At times they are used as “smart boxes” which join attachment points of a single device, such as an underwater connector or cable, to sensors, batteries, or other devices within the termination chamber.
When cables are joined to a junction box sometimes the cable-to-junction-box joint is connectorized, meaning that the cable does not actually penetrate the termination chamber, but instead is joined to it by way of a connector which passes through the termination chamber wall. In other common applications, the cable's bitter end itself actually penetrates the termination chamber. An underwater cable is essentially a long, thin pressure vessel in which there are voids, such as the interstices between twisted wire strands or between various other cable elements, which are nominally at one atmosphere pressure even when the cable is submerged to great depths. Prior-art termination chambers are typically filled with dielectric oil after the conductors within the chamber are attached to their respective attachment points. Oil is chosen due to the attributes that it is electrically isolative; it is able to evenly transmit the exterior ambient pressure to the interior portions of the chamber volume; and, it can be easily removed for maintenance and repair of the elements within the chamber. The oil pressure within the chamber is possibly very high depending upon its depth in the water. Therefore there can be a large pressure difference between the oil in the termination chamber and the cable interstices. Oil can undesirably flow into interstitial voids in the cable in case of nicks, pinholes, or other small perforations through the conductor jackets or, as described below, through boot seals.
Boot seals are frequently used to seal cable interfaces from the environment exterior to them. A generic example of a boot seal is shown in FIG. 4. Simple jacketed cable 36 in the example has a single, jacketed conductor 37. A first elastomeric boot-seal sleeve portion 38 is stretched over jacketed cable 36, and a second smaller elastomeric boot-seal sleeve portion 39 stretches over jacketed conductor 37. Pressure applied to the exterior of the FIG. 4 boot seal simply adds to the constrictive pressure of the elastomeric stretch, and enhances the seal between the boot seal and the elements of the cable. Boot seals such as shown in FIG. 4 are one-way seals. That is, they seal against intrusion of the exterior environment into the interface 40 between cable elements 36 and 37. But if there is an overpressure within cable interface 40, such as would occur with pressurized gas, water, or gel within interface 40, the overpressure could blow the boot seal axially away from the interface, or simply unseat sleeve 38 or 39.
Subsea cables are often so large that they cannot be practically transported to a place where they can be terminated in a laboratory-like environment; they must be terminated in the field. Therefore, there is often a need in the offshore industries for cable-to-connector junctions that can be installed, tested, and repaired in the field prior to immersion. Cables for subsea use commonly consist of an exterior jacket which houses a variety of individually jacketed electrical conductors and/or optical fibers within an inner core. The electrical conductors usually consist of stranded wires. In a typical termination process, the cable exterior jacket and core are cut back exposing lengths of the individually jacketed conductors. In the case of a cable carrying optical fibers within tubular conduits, tube-end-seal assemblies such as the example described in U.S. Pat. No. 6,321,021 to Cairns et al. (“the '021 patent”), the contents of which are included herein by reference, provide sealed barriers between the chamber volume and the interior portion of the fiber-carrying tubular conduits.
U.S. Pat. No. 5,577,926 to Cox (“the '926 patent”), the contents of which are incorporated herein by reference, describes a prior art arrangement. In the '926 patent, a cable is sealably joined mechanically to a junction box whose termination chamber houses the end of the cable core and the exposed conductors. The chamber is filled with dielectric oil. Boot seals are stretched across the joints between the core and the jackets of the exposed lengths of conductors thereby sealing those interfaces to prevent oil from entering the cable. The exposed conductors go further on within the oil chamber to eventually join to other conductors or to the attachment points of connectors or other devices. All joints between the conductors and attachment points are also sealed by boot seals. As a result, there are usually many seals and exposed, jacketed conductors within the oil-filled chamber. When completely installed, a compensator, such as a flexible portion of the chamber wall, allows the pressure within the oil to closely match that of the outside environment. Under pressure any perforations through the boot seals or conductor jackets will cause the chamber oil to be forced into the cable interstices. The chamber walls will then either collapse or rupture, allowing seawater to enter, and creating a catastrophic failure.
Another failure mode can occur when gel-filled cables are employed. In this mode, as cables are passed over handling devices such as pulleys, the gel can be “milked” toward the oil-filled termination chamber. That can unseat boot seals and result in subsequent failure. Still another failure mode, occurs when a cable is retrieved quickly from great depths. In this case pressurized gas expands within the cable, and seals within the oil-filled chamber can be temporarily or permanently unseated, allowing chamber oil to enter the cable interstices. Failure can also occur when the cable is under axial compression, as can happen during handling. In this case, if not arrested properly, the cable can piston into the oil-filled chamber and destroy the inner works.
U.S. Pat. No. 6,796,821 to Cairns et al. (“the '821 patent”), the contents of which are incorporated herein by reference, describes another prior art arrangement. Unlike arrangements prior to it, exemplified by the '926 patent, the '821 patent termination has separate first and second termination chambers intended, as discussed in the specification of the '821 patent, to obviate the aforementioned failure modes by providing an impenetrable barrier between a first chamber, and a second chamber which is filled with oil. The individual cable conductors are terminated in the first chamber to sealed penetrators which pass through the impenetrable barrier and onward into the second, oil-filled, chamber. The '821 patent discloses three basic embodiments: One with a first chamber filled with a cast, solid material, and maintained at one atmosphere pressure; one with a first chamber filled with a cast, solid material, and pressure compensated with grease to the ambient working pressure; and, one with a first chamber filled entirely with grease and compensated to the ambient working pressure. In all three embodiments the second chamber is oil-filled and pressure compensated to the ambient working pressure.
In the first two aforementioned embodiments of the '821 patent termination, once the conductors are terminated to attachment points on the impenetrable barrier the first chamber is filled with a pourable material which cures to a solid.
In the third aforementioned mentioned '821 patent embodiment, the first chamber is not filled with solid material, but rather is grease filled; and incorporates a compensator mechanism that balances the pressure within the grease to the ambient working pressure. Therefore, the first chamber in this third embodiment has all of the attributes of the earlier technology comprising only an oil-filled, pressure-balanced termination chamber.
In early art exemplified by the '926 patent, cable conductors extending outward from the core were exposed in a single oil-filled termination chamber and therein connected to the attachment points of connectors or other devices. All interfaces presenting potential leak paths of the oil into the cable were sealed with boot seals. The terminations were first completed up to the point of filling the chamber with oil. Next a method such as gas leak testing was employed to find any leaks from the chamber into the cable, or between the chamber and the exterior environment. If leaks were found, the termination could be easily dismantled for repair and retesting. Also at the point prior to oil filling, the quality of electrical and/or optical circuits could be tested, and if necessary, repaired. Even after oil filling, if defects were found in the termination, the oil could be drained, and repairs made. That is not the case with the '821 patent termination. Once the solid filler is installed into the first chamber, that portion can no longer be non-destructively disassembled for repair, nor can it be leak tested against water ingression. The solid filler embodiments of the '821 patent are, therefore, not completely testable or repairable in the field.
A subsea termination chamber has two fundamental requirements of a fill material: one, that it will transmit the ambient exterior pressure uniformly within the chamber; and two, that it will not escape. Filling the termination chamber with fluid satisfies the first of these requirements, but can fail the second.